Flies like yellow, bees like blue: how flower colours cater to the taste of pollinating insects


Hoverfly (Eristalis tenax) feeding on marigold.
Fir0002/Flagstaffotos, CC BY-NC

Jair Garcia, RMIT University; Adrian Dyer, RMIT University, and Mani Shrestha, Bayreuth UniversityWe all know the birds and the bees are important for pollination, and we often notice them in gardens and parks. But what about flies?

Flies are the second most common type of pollinator, so perhaps we should all be taught about the bees, the flies and then the birds. While we know animals may see colour differently, little was known about how fly pollination shapes the types of flowers we can find in nature.

In our new study we address this gap in our knowledge by evaluating how important fly pollinators sense and use colour, and how fly pollinated flowers have evolved colour signals.

Specialed flower visiting flies: a hoverfly (Eristalis tenax) (left panel), and a bee-fly (Poecilanthrax apache) (right panel)
Michael Becker, Pdeley

The way we see influences what we choose

We know that different humans often have preferences for certain colours, and in a similar way bees prefer blue hues.

Our colleague Lea Hannah has observed that hoverflies (Eristalis tenax) are much better at distinguishing between different shades of yellow than between different blues. Other research has also reported hoverflies have innate responses to yellow colours.




Read more:
The mystery of the blue flower: nature’s rare colour owes its existence to bee vision


Many flowering plants depend on attracting pollinators to reproduce, so the appearance of their flowers has evolved to cater to the preferences of the pollinators. We wanted to find out what this might mean for how different insects like bees or flies shape flower colours in a complex natural environment where both types of insect are present.

The Australian case study

Australia is a natural laboratory for understanding flower evolution due to its geological isolation. On the mainland Australian continent, flowers have predominately evolved colours to suit animal pollination.

Around Australia there are plant communities with different pollinators. For example, Macquarie Island has no bees, and flies are the only animal pollinator.

We assembled data from different locations, including a native habitat in mainland Australia where both bees and flies forage, to model how different insects influence flower colour signal evolution.

Measuring flower colours

Since we know different animals sense colour in different ways, we recorded the spectrum of different wavelengths of light reflected from the flowers with a spectrometer. We subsequently modelled these spectral signatures of plant flowers considering animal perception, allowing us to objectively quantify how signals have evolved. These analyses included mapping the evolutionary ancestry of the plants.

Generalisation or specialisation?

According to one school of thought, flower evolution is driven by competition between flowering plants. In this scenario, different species might have very different colours from one another, to increase their chances of being reliably identified and pollinated. This is a bit like how exclusive brands seek customers by having readily identifiable branding.

An alternative hypothesis to competition is facilitation. Plants may share preferred colour signals to attract a higher number of specific insects. This explanation is like how some competing businesses can do better by being physically close together to attract many customers.




Read more:
Plants use advertising-like strategies to attract bees with colour and scent


Our results demonstrate how flower colour signalling has dynamically evolved depending on the availability of insect pollinators, as happens in marketplaces.

In Victoria, flowers have converged to evolve colour signals preferred by their pollinators. The flowers of fly-pollinated orchids are typically yellowish-green, while closely related orchids pollinated by bees have more bluish and purple colours. The flowers appeared to share the preferred colours of their main pollinator, consistent with a facilitation hypothesis.

Typical flowers preferred by bees (Lobelia rhombifolia, left panel) and flies (Pterostylis melagramma, right panel) encountered in our study sites. Inserts show the spectral profile for each species as measured by a spectrometer.
Mani Shrestha

Our research showed flies can see differences between flowers of different species in response to the pollinator local “market”.

On Macquarie Island, where flies are the only pollinators, flower colours diverge from each other – but still stay within the range of the flies’ preferred colours. This is consistent with a competition strategy, where differences between plant species allow flies to more easily identify the colour of recently visited flowers.

When both fly and bee pollinators are present, flowers pollinated by flies appear to “filter out” bees to reduce the number of ineffective and opportunistic visitors. For example, in the Himalayas specialised plants require flies with long tongues to access floral rewards. This is similar to when a store wants to exclusively attract customers specifically interested in their product range.

Our findings on fly colour vision, along with novel precision agriculture techniques, can help using flies as alternative pollinators of crops. It also allows us to understand that if we want to see a full range of pollinating insects including beautiful hoverflies in our parks and gardens, we need to plant a range of flower types and colours.The Conversation

Jair Garcia, Research fellow, RMIT University; Adrian Dyer, Associate Professor, RMIT University, and Mani Shrestha, Postdoc & International Fellow, Disturbance Ecology, Bayreuth University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Decaying forest wood releases a whopping 10.9 billion tonnes of carbon each year. This will increase under climate change


Shutterstock

Marisa Stone, Griffith University; David Lindenmayer, Australian National University; Kurtis Nisbet, Griffith University, and Sebastian Seibold, Technical University of MunichIf you’ve wandered through a forest, you’ve probably dodged dead, rotting branches or stumps scattered on the ground. This is “deadwood”, and it plays several vital roles in forest ecosystems.

It provides habitat for small mammals, birds, amphibians and insects. And as deadwood decomposes it contributes to the ecosystem’s cycle of nutrients, which is important for plant growth.

But there’s another important role we have little understanding of on a global scale: the carbon deadwood releases as it decomposes, with part of it going into the soil and part into the atmosphere. Insects, such as termites and wood borers, can accelerate this process.

The world’s deadwood currently stores 73 billion tonnes of carbon. Our new research in Nature has, for the first time, calculated that 10.9 billion tonnes of this (around 15%) is released into the atmosphere and soil each year — a little more than the world’s emissions from burning fossil fuels.

But this amount can change depending on insect activity, and will likely increase under climate change. It’s vital deadwood is considered explicitly in all future climate change projections.

An extraordinary, global effort

Forests are crucial carbon sinks, where living trees capture and store carbon dioxide from the atmosphere, helping to regulate climate.
Deadwood — including fallen or still-standing trees, branches and stumps — makes up 8% of this carbon stock in the world’s forests.

Our aim was to measure the influence of climate and insects on the rate of decomposition — but it wasn’t easy. Our research paper is the result of an extraordinary effort to co-ordinate a large-scale cross-continent field experiment. More than 30 research groups worldwide took part.

White boxes on the forest floor
We used mesh cages to keep insects away from some deadwood to test their effect on decay.
Marisa Stone, Author provided

Wood from more than 140 tree species was laid out for up to three years at 55 forest sites on six continents, from the Amazon rainforest to Brisbane, Australia.
Half of these wood samples were in closed mesh cages to exclude insects from the decomposition process to test their effect, too.

Some sites had to be protected from elephants, another was lost to fire and another had to be rebuilt after a flood.

What we found

Our research showed the rate of deadwood decay and how insects contribute to it depend very strongly on climate.

We found the rate increased primarily with rising temperature, and was disproportionately greater in the tropics compared to all other cooler climatic regions.

In fact, deadwood in tropical regions lost a median mass of 28.2% every year. In cooler, temperate regions, the median mass lost was just 6.3%.

More deadwood decay occurs in the tropics because the region has greater biodiversity (more insects and fungi) to facilitate decomposition. As insects consume the wood, they render it to small particles, which speed up decay. The insects also introduce fungal species, which then finish the job.




Read more:
Wood beetles are nature’s recyclers – with a little help from fungi


Of the 10.9 billion tonnes of carbon dioxide released by deadwood each year, we estimate insect activity is responsible for 3.2 billion tonnes, or 29%.

Let’s break this down by region. In the tropics, insects were responsible for almost one-third of the carbon released from deadwood. In regions with low temperatures in forests of northern and temperate latitudes — such as in Canada and Finland — insects had little effect.

Mushrooms growing on a log
After insects break deadwood into smaller pieces, fungi are responsible for the final stages of decay.
Marisa Stone, Author provided

What does this mean in a changing climate?

Insects are sensitive to climate change and, with recent declines in insect biodiversity, the current and future roles of insects in deadwood are uncertain.

But given the vast majority of deadwood decay occurs in the tropics (93%), and that this region in general is set to become even warmer and wetter under climate change, it’s safe to say climate change will increase the amount of carbon deadwood releases each year.

Close-up of three termites in wood
Termites and other insects can speed up deadwood decay in warmer climates.
Shutterstock

It’s also worth bearing in mind that the amount of carbon dioxide released is still only a fraction of the total annual global deadwood carbon stock. That is, 85% of the global deadwood carbon stock remains on forest floors and continues to store carbon each year.

We recommend deadwood is left in place — in the forest. Removing deadwood may not only be destructive for biodiversity and the ability of forests to regenerate, but it could actually substantially increase atmospheric carbon.




Read more:
Photos from the field: zooming in on Australia’s hidden world of exquisite mites, snails and beetles


For example, if we used deadwood as a biofuel it could release the carbon that would otherwise have remained locked up each year. If the world’s deadwood was removed and burned, it would be release eight times more carbon than what’s currently emitted from burning fossil fuels.

This is particularly important in cooler climatic regions, where decomposition is slower and deadwood remains for several years as a vital carbon sink.

Lush, green forest
Deadwood is essential for a healthy forest ecosystem.
Milk tea/Unsplash, CC BY

What next?

The complex interplay of interactions between insects and climate on deadwood carbon release makes future climate projections a bit tricky.

To improve climate change predictions, we need much more detailed research on how communities of decomposer insects (such as the numbers of individuals and species) influence deadwood decomposition, not to mention potential effects from insect diversity loss.

But insect diversity loss is also likely to vary regionally and would require long-term studies over decades to determine.

For now, climate scientists must take the enormous annual emissions from deadwood into account in their research, so humanity can have a better understanding of climate change’s cascading effects.




Read more:
Trees can’t save us from climate change – but society will always depend on forests – podcast


The Conversation


Marisa Stone, Adjunct Research Fellow, Centre for Planetary Health and Food Security, Griffith University; David Lindenmayer, Professor, The Fenner School of Environment and Society, Australian National University; Kurtis Nisbet, Scientific Officer, Griffith University, and Sebastian Seibold, Adjunct Teaching Professor, Technical University of Munich

This article is republished from The Conversation under a Creative Commons license. Read the original article.